Acoustic positioning in a fading multipath environment

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Abstract
Acoustic positioning systems typically use the first acoustic arrival from each beacon to determine ranges to known beacon locations, and subsequent multipath arrivals are blanked out. As long as the first arrival is reliably present, these systems are dependable, but in many realistic positioning scenarios, the expected first arrival may be blocked by underwater obstacles or masked by noise. If a subsequent multipath arrival is mistaken for the missing first arrival, a position error will result. This paper presents a positioning system which uses the full multipath structure of the received signal to make the system robust to the fading of individual arrivals. The system determines its position by a two-step process. First, the detected arrivals in the multipath structure are identified with physical ray paths through the environment using a new algorithm which is robust to the disappearance of a subset of the predicted and measured arrivals. Then, the differences in arrival times between the measured arrivals and the predicted ray arrivals are used in a linear inversion to produce a position estimate. The operation of the system is simulated in a typical coastal environment where arrivals become unexpectedly absent due to blocking by unknown bottom topography, and where a high ambient noise level often produces missed arrivals and false detections.

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